The present invention relates to a circuit and method of controlling the average power delivered to an infrared (IR) source or emitter used in a non-dispersive infrared detection cell (NDIR cell),
An NDIR (non-dispersive infrared) cell is used to measure the quantity of a selected gas by means of IR absorbance. The main components of the cell are the infrared source or emitter, the sample chamber or light tube, a wavelength filter, and an infrared detector.
The IR signal from the emitter is typically modulated in some fashion so that thermal background signals can be removed from the desired measurement signal. Two methods often employed to modulate the emitter are to either mechanically chop the light or alternatively to pulse the electrical power delivered to the emitter. For the pulse modulation case, the electrical power supplied to the emitter is modulated at a frequency that causes the emitter to heat and cool thereby modulating the optical IR power emitted from its surface. The thermal response of the emitter must be sufficiently fast such that the depth of temperature modulation is adequate for the desired sensitivity at the selected modulation frequency.
One means to pulse power to the emitter is to pulse a voltage level to the emitter. This is referred to as the primary modulation. Usually, a 50% primary modulation is used where the emitter on-time and off-time are equal. During the off-time, the applied voltage can be a low voltage or zero volts. Preferably a non-zero voltage is used during the off-time to minimize the stress to the emitter frame during the pulsing. The primary modulation frequency can be selected to optimize signal-to-noise ratio of the NDIR while providing adequate response time for the bandwidth of the expected analyte signal. The instantaneous electrical power during the on-time is dependent upon the voltage applied to and the resistance of the emitter according to the following standard power equation:
  Power  =            Voltage      2        Resistance  
Often, the emitter resistance has a wide tolerance due to limitations in the manufacturing process, etc. If a constant voltage is pulsed to the emitter, a correspondingly wide range of power will be experienced. Since the sensitivity of the NDIR cell is dependent, among other things, upon the electrical power delivered to the IR emitter, a wide range of NDIR cell sensitivities will ensue. One means to minimize this variable sensitivity is to control the power to each IR emitter.
Since the resistance cannot be controlled by the end user, one option is to vary the emitter voltage level applied during the on-time to achieve constant power. This solution can be performed either with analog hardware, requiring a hardware control loop to be tuned for adequate response performance and stability, or it can be implemented using a digital control loop with a digital-to-analog (DAC) converter to adjust the voltage level. Because it may also be desirable to set the voltage to a non-zero value during the off-time, some means must be designed to allow the emitter voltage to switch between two voltage levels.